This invention relates to pressure sensing devices and the fabrication thereof.
In a paper entitled xe2x80x9cA miniature self-aligned pressure sensing elementxe2x80x9d by Goustouridis, Chatzandroulis, Normand and Tsoukalas (J. Micromech. Microeng. 6 (1996) pp 33 to 35), the entire contents of which are incorporated herein by reference, there is disclosed a fabrication technique for the production of a capacitive-type pressure sensor. Although this is a simple fabrication process which secures self-alignment and requires the use of only three masks, for no readily apparent reason sensors produced by the process were found to be unreliable in terms of long term stability.
A primary objective of at least certain aspects of the present invention is to provide an improved pressure-responsive device having reliable long term stability.
Other objectives of certain aspects of the present invention include the provision of:
a fabrication process for a capacitive single crystal silicon sensor with good characteristics in terms of long-term stability;
a fabrication process that may be based on a self-aligned technique which, if combined with thin film stress engineering, permits the fabrication of flat or deflected silicon diaphragms according to application; and
a fabrication process for a pressure switch based on the ohmic contact of two electrodes namely of the mobile and immobile electrodes made using a similar self-aligned process.
According to one aspect of the present invention there is provided a capacitive pressure-responsive device comprising first and second electrode structures having opposed surfaces with a spacing layer therebetween, the spacing layer being formed with a window which defines a cavity between the two electrode structures to allow the spacing therebetween to vary in dependence upon external pressure, characterised in that the window is produced with side walls which are substantially free of any localised undercut region in the vicinity of the interfaces with the adjacent electrode structures.
According to a second aspect of the present invention there is provided a process for the production of a capacitive pressure-responsive device comprising first and second electrode structures having opposed surfaces with a spacing layer therebetween, the spacing layer being formed with a window which defines a cavity between the two electrode structures to allow the spacing therebetween to vary in dependence upon external pressure, characterised in that the window is formed by anisotropic etching.
The window may be formed after the formation of the spacing layer.
Anisotropic etching will be understood to refer to an etching process which attacks the substrate, e.g. a silicon wafer, essentially in one direction only. Thus, by anisotropic etching it is possible to etch an aperture in a substrate such that the walls of the aperture are substantially perpendicular to the surface of the substrate. Ion assisted processes are particularly effective in securing highly anisotropic etching, resulting in a vertical profile (see Pages 199, 200 of VLSI Technology by S. M. Sze, McGraw Hill).
By forming the window by anisotropic etching, it is possible to obtain window side walls which are substantially free of any localised undercut region in the vicinity of the interface with the electrode on which the spacing layer is provided.
Through the elimination of such localised undercut region or regions, we have found that reliable long term stable performance of the sensor may be secured. The process of Goustouridis et al involves producing the window by means of an HF solution. We have discovered that the use of an HF solution, viz. an isoptropic technique, results in the formation of undercut regions in the vicinity of the window side wall/electrode interface and that this effect is responsible for the unreliable long term stability of sensors produced when following the teaching of Goustouridis et al.
The arrangement may be such that one electrode structure is substantially rigid or immobile while the other electrode structure is capable of flexing under applied external pressure whereby the spacing between the two electrode structures across the cavity varies with the applied external pressure.
The device may be fabricated as a switch device giving an output in response to the applied pressure attaining a predetermined threshold value, or as a sensor device giving an output which varies with the applied external pressure.
The spacing layer is preferably provided on the flexible electrode structure and may be formed integrally therewith.
The etching technique may comprise a drying etching technique, preferred examples of which are anisotropic plasma etching and anisotropic reactive ion etching.
Each electrode structure may be formed by a wafer of semiconductor material such as silicon, e.g a n-type or p-type silicon wafer. Thus, for instance, one electrode may comprise p-type silicon material and the other may comprise n-type silicon material. However, we do not exclude the possibility of one or both electrodes being formed of other conductive material such as a metal, e.g. the immobile electrode may be of metal.
The relatively flexible electrode structure may comprise an integrally formed structure made from a flexible highly conductive single crystal silicon by selectively dissolving part of the wafer away to leave a relatively highly doped region forming the electrode. The silicon may be doped with any element, e.g. boron, or combination of elements so that a relatively highly doped part of the silicon wafer exhibits an etching selectivity with respect to lightly doped or non-doped silicon. Boron is the preferred dopant.
Doping of the silicon, e.g. with boron, may be effected by doping through said window with vertical walls, the spacing layer serving during the doping step to restrict the doping primarily to the zone of the semiconductor wafer in registry with the window.
The doping step may be carried out in such a way that the dopant also penetrates laterally beyond the perimeter of the window so that there is some degree of overlap between the resulting flexible electrode and the spacing layer. The degree of overlap is generally comparable with the thickness of the flexible electrode.
The fabrication process of the invention may be carried out using only three masks for the complete fabrication of the wafer assembly including preparation for a metallization step. Also, as in Goustouridis et al, precise alignment is not necessary between the two bonded wafers during the fabrication.
The doped flexible electrode may be provided with an electrically insulating film on one or its both sides.
That surface of the less flexible electrode which is bonded to the spacing layer may be provided with a thin electrically insulating film to avoid electrical shorting of the capacitance between flexible and fixed electrode.
The less flexible electrode may be provided with a thick stress compensating layer on that surface remote from the spacing layer. This is particularly suitable for, but is not restricted to, applications requiring the fabrication of flexible diaphragms that should be flat, for instance for use in the pressure measurement of the body by the tonometric technique.
The cavity may contain gas which may, if desired, be pressurised, or it may be at least partially evacuated, e.g. substantially totally evacuated.
Any one of the electrically insulating films applied to the surfaces of the semiconductor wafers may be such as to resist high temperature treatment, e.g. a thermally grown oxide.